Semiconductor devices have recently become more highly integrated and smaller in size than ever before, which generated needs to develop sophisticated materials and processes for forming thin films in the fabrication of semiconductor devices.
A ferroelectric BLT layer is known to have better performance characteristics as compared with other ferroelectric layers. For example, a strontium bismuth titanate (SBT, SrBi2Ti2O9) layer requires a high crystallization temperature of 800° C. or higher, and a lead zirconate titanate (PZT, PbZrxTi1-xO3(0<x<1)) layer exhibits early fatigue only at 106 cycles. In contrast, it is reported that a BLT layer needs a relatively low crystallization temperature of around 700° C., and that it exhibits no sign of fatigue even at 1010 cycles (see [B. H. Park, B. S. Kang, S. D. Bu, T. W. Noh, J. Lee, and W. Jo, Nature, 401(14), 682(1999)]).
Of various methods of preparing such ferroelectric thin films, MOCVD is the most preferred for the reasons that: it can be carried out at a relatively low temperature; the composition and deposition rate of the thin film can be readily controlled by changing the amounts of the source materials and the carrier gas; and the deposited thin film has good uniformity, excellent conformal step coverage and improved hole filling property without damaging the surface of the substrate.
Generally, a precursor for MOCVD is required to have such properties as high thermostability, non-toxicity, a high vapor pressure and a high deposition rate. Especially preferred is a MOCVD mode carried out by direct liquid injection (DLI) of a soluble precursor dissolved in a solvent. In case of forming a ferroelectric thin layer by DLI, it is preferred that the corresponding metal precursors do not undergo chemical reactions in solution.
However, among conventional organometallic compounds applied to MOCVD for a BLT thin layer, a representative Ti precursor, Ti(i-OPr)4(titanium tetraisopropoxide), is very sensitive to moisture, and a representative La precursor, La(tmhd)3(lanthanum tetramethylheptanedionate), exhibits a very low solubility in an organic solvent.